Method for controlling a laundry drying machine with heat pump system and laundry drying machine controlled by such method

ABSTRACT

A laundry drying machine with a heat pump system comprises a process air circuit including a rotating drum, a blower and a heater, a refrigerant circuit including a compressor, a condenser, an expansion device, an evaporator, the condenser and evaporator being in heat exchange relationship with the process air circuit, an auxiliary condenser cooled by an air flow driven by a fan, and at least two temperature sensors placed in the process air circuit and/or in the refrigerant circuit. A method for controlling the laundry drying machine includes receiving an input indicating a desired behavior of the laundry drying machine selected from the group consisting of optimized use of energy, overall drying time and fabric care, and controlling components of the machine according to signals from the two temperature sensors and according to the desired behavior.

RELATED APPLICATION

This application claims the priority benefit of European Patent Application 12177506.8 filed on Jul. 23, 2012, the entirety of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to methods for controlling a laundry drying machine with a heat pump system comprising a process air circuit including a rotating drum, a blower and a heater; a refrigerant circuit including a compressor, a condenser, an expansion device, an evaporator, the condenser and evaporator being in heat exchange relationship with the process air circuit, and an auxiliary condenser cooled by an air flow driven by a fan; and at least two temperature sensors placed respectively in the process air circuit and in the refrigerant circuit.

BACKGROUND

The so called “hybrid” heat pump dryers, in which the process air is heated either by the condenser of a refrigerant circuit and by an auxiliary heater are well known in the art. Moreover, a hybrid heat pump dryer having an auxiliary condenser with an auxiliary fan (cooled by ambient air) is known from European Patent Application EP 999302.

Usually such hybrid heat pump dryers, despite being very efficient in term of use of energy, offer to the user only a quite limited range of choices for the drying process, for instance degree of final humidity content of laundry, or long or short drying cycle. Such few and simple choices can on one hand limit the operational ranges of the machine, and on the other hand limit the possible choices of the users which may depend on several factors.

SUMMARY

The purpose of this disclosure is therefore a goal oriented control method, which increases the choices of the user, and which can particularly optimize a choice on low energy consumption, on cycle overall time, or on fabric care of a hybrid heat pump household tumble dryer, with an optimized balance between heating and cooling power.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the disclosed methods and laundry dryers according to the disclosure will become clear from the following detailed description, with reference to the attached drawings in which:

FIG. 1 is a schematic view of a hybrid heat pump tumble dryer;

FIG. 2 is a block diagram showing a dual loop control architecture according to the disclosure;

FIGS. 3-5 are examples of control implementations according to different choices of the user based on energy strategy, time strategy and fabric care strategy, respectively;

FIG. 6 is a block diagram showing a prior-art control loop;

FIG. 7 is a diagram showing the temperature and residual moisture content behavior in a dryer using the prior-art control system of FIG. 6; and

FIGS. 8-10 are diagrams showing example energy optimized cycle temperature behaviors, time optimized cycle temperature behaviors and fabric care optimized cycle temperature behaviors, respectively.

DETAILED DESCRIPTION

With reference to FIG. 1, the process air circuit is the one that involves the evaporation of the water retained by the fabric and it is made up of a rotating drum 10 actuated by an electric motor and containing a certain amount of clothes, a process air blower 12 that sets the circuit process airflow, a condenser 14 and an heating element 16 that heat the air going inside the drum 10, an evaporator 18 where the moisture contained in the process air can condense, an auxiliary condenser 20 (sub-cooler), a compressor 22, an expansion tube 23, and a fan 26 for cooling the auxiliary condenser 20 with ambient air.

The clothes dryer comprises also a NTC temperature sensor T1 placed on the process air exhaust from the drum 10, and a NTC sensor T2 placed in the refrigerant circuit downstream from the compressor 22, the temperature sensors T1 and T2 being connected to a control process unit 28 that drives all components of the clothes dryer according to certain processes.

The clothes dryer can also include also other components, for example, an accumulator upstream of the compressor 22, which is not shown in FIG. 1 for sake of clarity.

An air channel conveys the process air to the evaporator 18, where the vapor contained in the air condenses due to the low temperature.

The heat pump circuit is the one that involves the refrigerant that with its phase variation transfers heat to the air circuit. The temperature sensor T2 that measures the refrigerant temperature may alternatively be placed in a position different from compressor outlet, for instance in the capillary tube or other places. The auxiliary fan 26 increases the heat exchange on the auxiliary condenser 20.

Also the temperature sensor T1 may be placed in a different position than the one shown in FIG. 1, but in any case it is placed in the moist air circuit, by the drum outlet, the blower 12 or the evaporator 18.

The second temperature sensor T2, instead of being placed in the refrigerant circuit, may alternatively be placed in the moist air circuit in a position different from the first sensor T1, for instance by the heater 16, the condenser 14, or the auxiliary condenser 20.

The controllable variables of the system, which may be continuously adjusted or simply ON/OFF, are the compressor speed, the process air blower speed, the heating element power and the auxiliary fan speed.

Those variables are controlled in order to provide and remove the right quantity of energy respectively by means of the condenser 14 plus the heating element 16 and the evaporator 18, by compromising between evaporation and condensation.

In the most of the cases, to reduce the cost of the system, the motors of the compressor 22 and the process air blower 12 are constant speed motors, therefore it is not possible to change their speeds.

According to a common practice of controlling the clothes dryer, the compressor 22 is kept on for the entire drying cycle while the heater 16 switches on/off in order to manage the temperature of the tumble dryer by feeding back the drum exhaust temperature measured by sensor T1. Indeed, the drum output temperature is usually a good approximation of the clothes temperature which is therefore kept under control.

Since it is required that the compressor stay on, due to inefficiency in turning off and on the heat pump system, and to prevent shifting in the working point of the system that would result in less energy removed in the evaporator 18, thus less condensation and overheating of the compressor 22, its temperature has to be controlled. Therefore when the temperature of the compressor 22, sensed by sensor T2, reaches a certain value close to the high limit temperature switch off, the auxiliary fan 26 is turned on.

The feedback is usually made through hysteresis control, i.e. the heater 16 and the auxiliary fan 26 are switched on when the feedback temperature is below a predefined threshold and switched off when it is above a second predefined threshold.

Up to now we have described a dryer which can be controlled either according to prior art or according to the disclosure. As a matter of fact the main drawback of the known control system is the difficulty in creating a customized appliance behavior aiming to optimize system performances according to customer choices, who may desire to save energy, to save time or alternately to prefer a more gentle treatment of clothes, for instance by keeping the drying temperatures lower.

The methods according to the disclosure can control every component of the clothes dryer, and preferably both the auxiliary cooling fan 26 and the heater 16 of a tumble hybrid heat pump dryer, optimizing alternatively energy consumption, drying time or fabric care according to a selection made by the user by means of a user interface 30. This selection can be done through a button, touch display, cycle selection, etc.

Once the user has made his/her selection, the system temperatures can be controlled by means of several actuators for the auxiliary cooling fan 26, the heater 16, the compressor 22, and the process air blower 12. The way all these actuators are used affects the overall system performances in terms of energy consumption, cycle duration, water extraction efficiency, final moisture retention at the end of the cycle, fabric care (wrinkles, shrinkage, etc.), etc..

The present disclosure provides therefore methods of choosing how to use these actuators in the different parts of a drying cycle.

The disclosure is effective even in the case of one or more of the actuators cannot be continuously controlled, e.g. fixed speed compressor, fixed speed fan, etc.

According to the disclosure, the drying cycle is conceptually divided in three phases of variable duration: warm up (WU), mid phase (MP) and cool down (CD). In the following descriptions, the three phases will be identified by means of two temperature measurements and cycle length.

In particular, naming:

t₀=0 the beginning of the cycle,

t_(end) the time at the end of the cycle,

t₂₀=0.2*t_(end),

t₅₀=0.5*t_(end),

t₇₀=0.7*_(tend),

t₈₀=0.8*t_(end),

T₁ _(—) _(start) the value of temperature T₁ measured at time t₀,

T₁ _(—) _(mid) the maximum value of temperature T₁ measured from t₀ to t₅₀,

T₁ _(—) _(threshold)=(T₁ _(—) _(mid)−T₁ _(—) _(start))*0.8+T₁ _(—) _(start),

t_(r1) the first time at which the temperature T₁ is greater than T₁ _(—) _(threshold),

T₂ _(—) _(start) the value of temperature T₂ measured at time t₀,

T₂ _(—) _(mid) the maximum value of temperature T₂ measured from t₀ to t₅₀,

T₂ _(—) _(threshold)=(T₂ _(—) _(mid)−T₂ _(—) _(start))*0.8+T₂ _(—) _(start),

t_(r2) the first time at which the temperature T₂ is greater than T₂ _(—) _(threshold),

t_(WU)=min(t20, tr1, tr2)

t_(MP) _(—) start=max(t₂₀, t_(WU)*1.2)

t_(MP)_end=t₇₀

_(tCD) _(—) _(start)=t₈₀

The following definitions of the three phases of the cycle are given:

Warm up (WU): starts at time t₀ and ends at time t_(WU)

Mid phase (MP): starts at time t_(MP) _(—) start and ends at time t_(MP) _(—) _(end)

Cool down (CD): starts at time t_(CD) _(—) _(start) and ends at time t_(end)

Moreover, naming:

P_(WU) the average power absorbed by the heating element during WU phase

P_(MP) the average power absorbed by the heating element during MP phase

P_(CD) the average power absorbed by the heating element during CD phase

S_(F) _(—) _(WU) the average speed of the auxiliary fan during WU phase

S_(F) _(—) _(MP) the average speed of the auxiliary fan during MP phase

S_(F) _(—) _(CD) the average speed of the auxiliary fan during CD phase

S_(C) _(—) _(WU) the average speed of the compressor during WU phase

S_(C) _(—) _(MP) the average speed of the compressor during MP phase

S_(C) _(—) _(CD) the average speed of the compressor during CD phase

S_(B) _(—) _(WU) the average speed of the process air blower fan during WU phase

S_(B) _(—) _(MP) the average speed of the process air blower fan during MP phase

S_(FB) _(—) _(CD) the average speed of the process air blower fan during CD phase

In case of discrete control the averages are computed taking in account 0 as OFF and 1 as ON.

The disclosed controller 28 will be provided with the possibility to operate in at least two of the following cycles based on a selection made via the user interface 30, to which the following values of parameters apply:

Energy Optimized Cycle, characterized by having:

P_(MP)<0.2*P_(WU), P_(CD)<0.2*P_(WU)

S_(f) _(—) _(WU)<0.25*S_(F) _(—) _(MP, S) _(f) _(—) _(CD)>S_(F) _(—) _(WU)

S_(C) _(—) _(CD)=<S_(C) _(—) _(MP, S) _(C) _(—) _(CD)=<S_(C) _(—) _(WU)

S_(B) _(—) _(WU)=<S_(B) _(—) _(MP)=<S_(B) _(—) _(CD)

Time Optimized Cycle, characterized by having:

0.5*P_(WU)<P_(MP)<1.2*P_(WU), P_(CD)<1.2*P_(WU)

S_(f) _(—) _(WU)<0.25*S_(F) _(—) _(MP), S_(f) _(—CD) >S_(F) _(—) _(WU)

S_(C) _(—) _(CD)=<S_(C) _(—) _(MP), S_(C) _(—) _(CD)=<S_(C) _(—) _(WU)

S_(B) _(—) _(WU)=<S_(B) _(—) _(MP)=<S_(B) _(—) _(CD)

Fabric Care Optimized Cycle, characterized by having:

0.2*P_(WU)<P_(MP)<0.5*P_(WU),PCD<0.25*P_(WU)

S_(f) _(—) _(WU)<0.25*S_(F) _(—) _(MP), S_(f) _(—) _(CD)>S_(F) _(—) _(WU)

S_(C) _(—) _(CD)=<S_(C) _(—) _(MP), S_(C) _(—) _(CD)=<S_(C) _(—) _(WU)

S_(B) _(—) _(WU)=<S_(B) _(—) _(MP)=<S_(B) _(—) _(CD)

Of course the above parameter values are only examples and they can change depending on the actual dryer in which the methods according to the disclosure are implemented.

A conceptual scheme which is shown in FIG. 2, changes both auxiliary fan motor speed and heating power according to two temperature measurements by the sensors T1 and T2, thus controlling the energy delivered to the load inside the drum 10 and the energy removed from the refrigerant giving the possibility to optimize different system performance objectives.

One example of the possible implementations of the control strategy shown in FIG. 2, considering for sake of simplicity that the process air blower 12 and compressor 22 are maintained at a constant speed during the cycle, for the energy, the time and the fabric care strategy are respectively drawn in the FIGS. 3-5. In the examples of FIGS. 3-5, the temperature sensed by sensor T1 is the drum outlet temperature while the temperature sensed by sensor T2 is the capillary temperature of the refrigerant circuit.

The control strategy according to the disclosure has been compared with a simple known strategy in which the hysteresis on T1 controls the heater actuation while the hysteresis on T2 controls the fan actuation, as shown in FIG. 6, referred to a drying cycle of a 4 kg load.

With the control system shown in FIG. 6, example results are shown in FIG. 7, which shows an energy consumption of 1.69 kWh and a drying time around 92 minutes. In the diagrams, reference A indicates the temperature of process air entering the drum 10, reference B indicates the temperature of air measured at the exhaust of the drum 10, reference C indicates the capillary temperature of the refrigerant circuit, and reference D indicates the residual moisture content of the fabric inside the drum 10.

The example energy optimized cycle shown in FIG. 8 (corresponding to the example control scheme of FIG. 3), reveals a lower energy consumption around 1.54 kWh (−9%) and a drying time around 98 minutes (+8%) compared to the control system of FIGS. 6 and 7.

The example time optimized cycle shown in FIGS. 4 and 9 has a comparable energy consumption 1.72 kWh (+2%) and a comparable drying time, around 90 minutes (−1%).

The example fabric optimized cycle shown in FIGS. 5 and 10 keeps the fabric temperature low and avoids the temperature increase at the cycle end and therefore reduces the stress on the fabric. In terms of performances, the energy absorbed is slightly below the reference cycle of FIGS. 6 and 7, i.e. 1.6 kWh (−5%) but the drying time is increased lasting 118 minutes (+29%). 

1. A method for controlling a laundry drying machine with a heat pump system comprising a process air circuit including a rotating drum, a blower and a heater, a refrigerant circuit including a compressor, a condenser, an expansion device, an evaporator, the condenser and evaporator being in heat exchange relationship with the process air circuit, an auxiliary condenser cooled by an air flow driven by a fan, and at least two temperature sensors placed in the process air circuit and/or in the refrigerant circuit, the method comprising: receiving an input indicating a desired behavior of the laundry drying machine selected from the group consisting of optimized use of energy, overall drying time and fabric care, and controlling components of the laundry drying machine according to signals from the two temperature sensors and according to the desired behavior.
 2. A method according to claim 1, wherein at least two components of the laundry drying machine are controlled.
 3. A method according to claim 2, wherein the at least two components to be controlled are selected from the group consisting of blower, heater, compressor and fan.
 4. A laundry drying machine comprising: a heat pump system comprising a process air circuit including a rotating drum, a blower and a heater, a refrigerant circuit including a compressor, a condenser, an expansion device, an evaporator, the condenser and evaporator being in heat exchange relationship with the process air circuit, an auxiliary condenser cooled by an air flow driven by a fan, at least two temperature sensors placed in the process air circuit and/or in the refrigerant circuit, a user interface configured to allow a user to choose a desired behavior of the laundry drying machine selected from the group consisting of optimized use of energy, overall drying time and fabric care, and a control unit coupled to the user interface and configured to control components of the laundry drying machine according to signals from the two temperature sensors and according to the desired behavior.
 5. A laundry drying machine according to claim 4, wherein at least two of the components are adapted to be controlled.
 6. A laundry machine according to claim 5, wherein the at least two components to be controlled are selected from the group consisting of the blower, the heater, the compressor, and the fan. 